Under the current circumstances, as loosening prevention threaded fasteners to be used for parts in which loosening may occur due to the influence of vibrations or external forces, there have been known one with an adhesive agent coated on a male or external screw thread (refer to a first patent document), and another one which is constructed such that a male screw thread is deformed to generate torque resistance against loosening (refer to a second patent document).
Among these, in the first-mentioned one in which an adhesive agent is coated on a male screw threaded portion, a step for coating the adhesive agent to the screw thread is separately added in a screw thread production process or in a fastening process, and hence, a product cost and a work cost (in the case of coating being carried out before fastening) together with the cost of the adhesive agent itself will rise.
In addition, the adhesive agent includes a resin as a main component, and so, there arises a problem that the fixing or adhesive force is reduced due to the softening of the resin at the time of high temperature, and a certain amount of time is required from the time of fastening to the time at which the adhesive agent solidifies, thereby resulting in a long working time.
Besides, the adhesive agent is coated on a threaded fitting portion, so the coefficient of friction of the threaded portion at the time of fastening changes to a large extent, and it is necessary to adjust the tightening or bolting condition at the time of fastening, and hence, there is a need to carry out special handling which is not required in common threaded fasteners.
Further, at the time of tightening operation, exfoliated or peeled powder of the adhesive agent becomes coarse particulates, powder dust, etc., which fall into surroundings of a working area, thus making it necessary to deal with these particulates or powder dust.
On the other hand, the threaded fastener of the second patent document is constructed such that a slit is formed from a crest of the male screw thread, and a pressure side screw thread half portion is forced into pressure contact with a pressure flank surface of the female screw thread, and at the same time a clearance side screw thread half portion is forced into pressure contact with a clearance flank surface of the female screw thread.
The shaping of such a male screw thread is carried out as follows. First, a thread groove for slitting (hereinafter slit forming thread groove), which is to be made into a slit, and a thread groove for a product (product-use thread groove), which is to be made into a product groove, are formed, and subsequently, plastic deformation is carried out is such a manner that the product-use thread groove is expanded and the slit forming thread groove is narrowed so as to mound an area between adjacent groove portions of the product-use thread groove thereby to form a male screw thread, while leaving a slit in a screw thread crest.
However, the screw thread half portion of the male screw thread has high rigidity, and will deform the pressure flank surface and the clearance flank surface of the female screw thread to a large extent, so that the strength of the threaded fitting portion becomes unable to endure with respect to a tightening force at the time of fastening, and destruction of the fitted screw thread will occur at a low level of axial force or tension. For that reason, there is a problem that it is difficult to obtain high axial tension at the time of tightening, and a slide occurs in a fastening portion between the male screw thread and the female screw thread, at a location to which an external force is applied after tightening.
In addition, the fastening is carried out by causing the pressure flank surface of the screw thread, which becomes a tightening surface, to [[ ]] be deformed to a large extent, and hence, the axial tension generated by the same torque becomes unstable.
Moreover, the female screw thread is plastically deformed at the time of fastening, and hence, when it is repeatedly used, an initial torque resistance against a loosening direction will not be able to be expected, and the male screw will be rotated in a returning direction with a slight torque.
Further, for production, there are required two form rolling dies, i.e., a form rolling die for forming a slit forming thread groove and a product-use thread groove, and another form rolling die for forming a final male screw thread. Even if they are combined or integrated into one or single form rolling die, die portions for respective processes will be required for the single form rolling die, and so, special or exclusive equipment is required, separately from production equipment for ordinary screw threads, thus leading to a cost rise.
On the other hand, in recent years, technological innovation with respect to parts weight saving progresses, and the form of female screws and their construction methods are also changing, and so nuts are not used, but parts to which female screw processing is directly applied to members are increasing in number. For this reason, in many cases, due to the dimensional restrictions of a member in which a female screw is formed, the threads per inch of a female screw cannot be ensured enough, and the threads per inch thereof fitted with a male screw are decreasing in comparison with a nut. In contrast to this, in the conventional parts or fasteners as mentioned above, if the fitting length thereof at the same level of a nut is not secured, it is difficult for parts to exhibit a locking effect, and the fitted threads per inch cannot be ensured enough, so there is a limitation in applying them to female screws.
Further, many of these female screws also have been fabricated by means of rolling processing or component rolling, and a member hole diameter before component rolling will affect a female screw thread inside diameter after the rolling, so that a thread form of an inside diameter portion will be a mountain shape with a depression in its central portion.
In addition, according to the shape of a hole peripheral part before the rolling, variation may occur in the inside diameter, and in such a case, the fitting state of the male screw and the female screw will become still more unstable. With a male screw part or fastener in which an axial force or tension is not stable, as referred to above, a situation will occur where a locking effect cannot be exhibited.